This invention relates to control systems, and particularly, though not exclusively, to aircraft flight-control systems.
This invention is particularly concerned with control systems of the kind in which data feed back from the control output of the system are compared with the input data supplied to the system to detect any difference between the actual output and the desired output as represented by the input data. The system modifies the output as necessary to maintain the difference as small as possible, so that the output varies in accordance with the input.
An example of the use of a control system incorporating feedback is the servocontrol of the position of an aircraft's flight-control surfaces (for example, the elevators) in accordance with the pilot's operation of the control column. For control of the elevators, the position of the control column in the fore and aft direction is sensed by transducers coupled to the column, and input signals representative of this position are supplied by the transducers to respective control computers.
The control computers also receive feedback signals from transducers sensing the position of the elevators and compare these feedback signals with the input signals to determine any error between the elevator position demanded by the pilot and the actual elevator position. An output signal is generated by each control computer for application, in normal circumstances, to a respective actuator to move the elevators as required to reduce the error. For reasons of safety, it is common practice to provide three or more control computers and respective associated transducers so as to provide a degree of redundancy to ensure continued correct control in spite of the occurrence of a fault or failure in the system. If the output signal of one of the control computers differs from those of the other computers by a larger amount than would be expected on the basis of known manufacturing and operating tolerances, that computer is regarded as being faulty, and is prevented from continuing to contribute to the control of the elevators.
In the past, the control computers of control systems have commonly been based on analogue processing techniques. In an analogue processing unit, each variable is represented by the magnitude of a parameter such as voltage or current. The magnitude of such a parameter can vary continuously over a range of values in proportion to the magnitude of the variable it represents. Each calculation takes place in a separate circuit specially designed to perform that calculation and permanently dedicated to it. Thus all the calculations involved in deriving the output control signal may be performed more or less concurrently (allowing for delays in signal transmission between circuits, for example), since for each calculation there is a circuit continuously available to perform the calculation and supply the result to the circuit for the next calculation. In general, therefore, the predominant factor limiting the speed of response of the output signal to changes in the input signal has been the propagation time of signals in the computer, rather than, for example, the total number of calculations and their sequence.
However, the application of digital computing techniques to control computers has altered the relative significance of possible limitations on the speed of computation. Digital computation involves assigning the magnitude of a variable to one of a finite number of discrete magnitude increments which together embrace the full range of possible magnitude of the variable. Each magnitude increment is identified by a respective and different numeral (generally in binary form) the magnitude of the numeral being indicative of the location, within the magnitude range, of the respective increment. The magnitude of a variable can thus be represented by the appropriate combination of binary digits, which is in turn represented electrically by a corresponding combination of voltages or currents each having one of two fixed magnitudes. Accurate storage and transmission of variables represented in this way can be effected more easily and reliably than is the case with variables represented in analogue form. Consequently, digital calculations are performed successively in a single, general-purpose calculation circuit, the data involved in each calculation and the results thereof being transmitted from and to a digital storage circuit. The operation of the calculation circuit is not limited to any particular calculation, but is adapted and controlled for each successive calculation by a program of instructions which are also retained in the storage circuit. This mode of operation permits great flexibility in the range of computation which can be performed by a digital processing unit since a modification, or even a complete substitution, of a computation can be accomplished merely by altering the program of instructions contained in electrical-signal form in the storage sircuit.
The calculation circuit can, however, handle only one calculation at a time, each calculation in the overall computation of, for example, an output command signal being completed and the result stored for use in subsequent calculations before the next calculation is commenced. Until a full cycle of calculations involved in a computation of an output signal is completed, there is no output signal available from that computation. In control computers it is common practice to retain each successive computer output signal in analogue form in a sample-and-hold circuit and supply it therefrom for the duration of the following computation cycle, until the result of that computation becomes available and can in turn be stored and supplied. Thus the speed of response of a control system incorporating digital computing techniques depends on the speed of operation of the calculation circuit, the time taken for exchange of data between the calculation circuit and the storage circuit, and the length and complexity of the sequence of calculations involved in computing the output signal.
If any input parameter upon which the calculations are based changes during a sequence of calculations, such change may not be manifested as a corresponding change in the output signal until the end of the next following sequence of calculations. Similarly, any cyclic variation in an output parameter with a period of the same order as, or shorter than, the period of one calculation cycle, cannot be followed (and therefore corrected accurately by the output signal).
In the case of aircraft flight-control, for example, it is desirable that any oscillation in the position of a flight-control surface should be detected and neutralized as quickly as possible. Such oscillation may be caused by `hunting` of an actuator about its correct position and, if this is not damped out by the flight-control system, may result in a dangerous instability in the aircraft's attitude. In known systems using digital processing techniques, the required speed of response has been achieved only by using very fast (and therefore, very expensive) digital processing equipment which can accomplish a full cycle of calculations in a period less than that of the fastest possible unwanted oscillation.